1. Field of the Invention
The present invention relates generally to a charge pump utilized in a phase locked loop, and more particularly to a charge pump having fast turn on and turn off response times.
2. Description of the Prior Art
A charge pump can be an important component when utilizing a phase locked loop (PLL) to obtain a specific frequency. Commonly a phase and frequency detector (PFD) receives as input a reference frequency signals and a divided frequency signals outputted by a voltage controlled oscillator (VCO). The PFD outputs an UP signal if the phase of the reference frequency signal leads the phase of the divided frequency signal or outputs a DOWN signal if the phase of the divided frequency signal leads the phase of the reference frequency signal.
The UP and DOWN signals are sent to control switches in the charge pump to cause the charge pump to act as a current source to a low pass filter or a current sink from the low pass filter according to the respective UP and DOWN signals. The positive or negative flow of current through the low pass filter controls the VCO to adjust its output frequency accordingly. The output of the VCO is transmitted through the frequency divider and back to an input of the PFD, completing the loop. The process continues until the phases of the reference frequency and the divided frequency are aligned.
Therefore, to a great degree, the reduction of noise as well as the performance of the entire PLL depends upon the response speed of the charge pump to switch between outputting or sinking a constant current regardless of variable loads according to the received UP and DOWN signals.
Please refer to FIG. 1, which shows a conventional charge pump 100 comprising a cascoded current mirror and switch on source node of a MOS switch. The current mirror is cascoded with high output impedance so that the current variation is less sensitive to the output voltage and retains a substantially constant current level regardless of loading.
The charge pump 100 comprises a first row of cascoded switches P7, P8, and P9 in order. The source of switch P7 is coupled to VDD and the drain of switch P9 is coupled to ground via a current source Icp. The gates of switches P7, P8, and P9 are coupled to ground, a node F, and a first input voltage Vb1 respectively.
A second row of cascoded switches P6, P3, P4, N3, N4, and N6 in order are also comprised by the charge pump 100 as are a third row of cascoded switches P5, P1, P2, N1, N2, and N5 in order. The source of switches P6 and P 5 are each coupled to VDD and the source of switches N6 and N5 are each coupled to ground. The gates of the second row of cascoded switches P6, P3, P4, N3, N4, and N6 are coupled to ground, the node F, the first input voltage Vb1, a second input voltage Vb2, a node H, and VDD respectively. The gates of the third row of cascoded switches P5, P1, P2, N1, N2, and N5 are coupled to a signal UPB, the node F, the first input voltage Vb1, the second input voltage Vb2, the node H, and a signal DN respectively. Node F is also coupled with a node G, which is in turn coupled between the drain of switch P4 and the drain of switch N3.
There are also capacitors C1 formed between VDD and a node E, which is coupled to the NODE F, and C2 formed between ground and the node H. The signals DN and UPB are derived from the UP and DOWN signals outputted by the PFD described earlier and are utilized to alternate the charge pump 100 between a current source and a current sink. UPB is an inverted version of the UP signal (UP BAR), so that when the signal UP goes from high to low, UPB goes from low to high and visa versa.
Switches P1-P9 function to cause the charge pump 100 to act as a current source and switches N1-N5 function to cause the charge pump 100 to act as a current sink. In these embodiments, switches P1-P9 are P-MOS transistors and switches N1-N5 are N-MOS transistors but a reversal of these P and N characteristics and accompanying adjustments is to be considered well within the scope of the present invention and present in other embodiments.
Please refer to FIG. 2 in conjunction with FIG. 1 for an example description of the operation of the charge pump 100. FIG. 2 is a timing diagram showing the relative voltages at the switches N1 and N2 as the charge pump 100 is switched from off state (no current flow out or in) to a current sink via the signal DN at the gate of switch N5. It should be noted that all indicated voltages and current values in FIG. 2, FIG. 4, and FIG. 6 are approximations given as examples only and actual results may vary depending on design considerations and manufacturing methods.
As the diagram in FIG. 2 shows, when the signal DN goes high, the switch N5 is turned on and the node D will be dragged down to 0 volts. Because the gate voltage at switch N2 maintains a relative high voltage, the voltage at the node D being reduced to 0 volts causes the switch N2 to turn on. The voltage at the source of the switch N1 then goes low because the switch N2 is turned on, causing the switch N1 to also turn on. With the switches N5, N2, and N1 all turned on, the charge pump 100 sinks current from the node 1, which may be coupled to a low pass filter, to ground. It should be apparent to one skilled in the art that a similar process is followed when the signal UPB goes from high to low at the gate of switch P5 as the charge pump 100 converts from being a current sink to being a current source.
The cascoded arrangement of the switches in the charge pump 100 essentially maintains a constant current level regardless of loading as desired. However, when the switch N5 is turned off, the node D become floating, which makes the switch N2 turn off very slow, which in turn causes the switch N1 to also turn off slowly. Please notice the circled portions of three of the waveforms in FIG. 2 that illustrate the slow rise of voltages at nodes D and C and the resulting very slow turn off of the output current from node 1. This ripple effect of having a faster transistor turning on a slower transistor, which in turn turns on a yet slower transistor, gives slow response and introduces unwanted noise into the charge pump.